This invention relates to an expansion device for a heat pump.
Heat pumps employ a compressor, an indoor heat exchanger, an outdoor heat exchanger, an expansion device and 4-way reversing valve, to switch operation between cooling and heating modes. Heat pumps utilize an expansion device through which the refrigerant flow expands from high pressure and temperature to low pressure and temperature. Different size restriction of the expansion device is required for proper system operation depending upon whether the heat pump is in a cooling or heating mode of operation. Obviously, when the system is operating in cooling or in heating mode, the direction of the refrigerant flow through the expansion device is reversed.
Prior art heat pump systems with single expansion devices use a moveable piston that moves in a first direction in which its flow resistance is substantially higher than when it is moved in an opposite second direction. The first direction corresponds to the heating mode and second direction corresponds the cooling mode. The piston is prone to wear, which adversely effects the operation and reliability of the system due to undesirably large tolerances and contamination. Furthermore, modern heat pump systems are incorporating alternate refrigerants, such as R410A, and POE oils. The system utilizing R410A refrigerant operate at much higher pressure differentials than more common R22 and R134A refrigerants employed in the past within the system. This adversely impacts the expansion device wear, lubrication and results in higher loads during transient conditions of operation.
Therefore, there is a need for a single reliable, inexpensive expansion device for the heat pump systems that is not as prone to wear and reliability problems.